In a typical clutch assembly for an automatic transmission (as illustrated in FIG. 8), a clutch pack 16 is provided for providing a frictional engagement between a first member 12 and a second member 14 which is rotatable relative to the first member 12. A piston 122 is provided within a fluid chamber 24 such that the application of fluid pressure to the fluid chamber 24 causes the piston 122 to move into engagement with the clutch pack 16 to frictionally engage the second member 14 to the first member 12. The contacting of the friction plates with increasing pressure eventually causes the rotation of the second rotatable member 14 which the system is designed to engage. A problem that has been recognized in the art for a typical clutch assembly of this type is that the centrifugal force of the fluid can put a positive pressure on the apply piston 122 as illustrated in FIG. 8. The pressure generated by the centrifugal force of the fluid can cause unintended engagement of the clutch pack 16.
Thus, a balance piston system has been developed as illustrated in FIG. 9 in which the balance piston 128 is disposed between the apply piston 122 and the clutch pack 16 so that hydraulic fluid is present on opposite sides of the apply piston 122. When the balance piston system is spinning, the balance piston 128 traps fluid at the outer edge of the cavity 130 that it creates with the apply piston 122 to counteract the centrifugal forces caused by the fluid on the other side of the apply piston 122 as illustrated in FIG. 9. The balance piston 128 is provided with a seal member 126 that is bonded to an outer edge 122B and contacts an inner surface of the axially extending arm 122B of the apply piston 122. This seal is critical for the proper functioning of the balance piston system. With the seal 126 applied to the outer surface of the balance piston 128, the complexity and cost of the manufacture of the balance piston 128 is significantly increased.
The present invention provides a balance piston/apply piston assembly that differs from typical assemblies in that the sealing elastomer (or other material) which is used to create the seal between the balance piston and the apply piston is bonded to the apply piston, not the balance piston as in typical assemblies. The fact that all of the rubber molding occurs on the apply piston, and none on the balance piston, provides a significant cost advantage over the typical assembly design where rubber must be molded to both components. With this invention, the balance piston's fabrication does not require adhesive or molding of rubber, and thus, no quality processes are required relating to a post-molded rubber inspection.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.